This invention relates to injection molding and more particularly concerns molding of elongated articles at least portions of which have extremely thin walls.
Various types of elongated thin wall molded plastic articles are presently manfuactured and used for many different purposes. Such articles include parts for ball point pens and various probe cover devices or sheaths. The widely employed electronic digital thermometer embodies a probe for insertion into a body cavity where temperature is to be measured. Removable probe covers or sheaths, either disposable or sterilizable, are provided for ready attachment to and detachment from the thermometer probe. Particularly for disposable devices, it is important to maintain a low cost. Nevertheless, because of limitations on molding techniques of the prior art, most elongated thin wall articles, such as thermometer probe covers, have been made in two parts. A first part is a relatively thick wall strong body portion. A second part comprises a thin wall tip that is required to afford rapid heat transfer from the exterior of the sheath to the interior of the sheath where the probe sensing element is positioned. Examples of such two-part probe covers are found in the U.S. Pat. Nos. to Mack et al, 2,983,385; Ensign et al, 3,349,896; 3,500,280 and 3,367,186; Keller, 3,469,449; and Outdewaall, 3,822,593.
A widely employed thermometer cover having a metal tip formed together with the molded plastic body is manufactured by the assignee of the present invention and described in the U.S. Pat. No. 3,719,396 to Van DeWalker et al. Two-part articles are more expensive.
Although the patent to Mueller et al, 3,729,998 suggests a sheath with a thin wall tip, there is no apparatus or method known in the prior art that can form such molded covers with tip walls sufficiently thin for a suitably high rate of heat transfer.
Despite the economic advantage derived from an integrally molded cover having a properly thin wall tip, prior art molding techniques are incapable of solving the many problems required for this type of fabrication. A typical thermometer cover has a length of about 4 inches, an average maximum diameter of considerably less than 1/2 inch and a wall having a thickness measured in thousandths of an inch. For commonly used injection molding materials such as polyethelene, for example, heat characteristics that are required to obtain an acceptably short response time dictate a wall thickness at the temperature sensing tip of about 0.020 inches. A preferable wall thickness of the tip is 0.010 inches.
Two major problems have prevented adequate solution of this thin wall molding problem in the past. These two problems are (a) maintaining proper relative position of a long, thin cantilevered core within a mold cavity and (b) the attendant problem of adequately and uniformly venting cavity gases during injection.
In this type of manufacture, material at a temperature of approximately 485.degree. is injected under pressures of many tons, typical injection pressure being significantly greater than ten thousand pounds per square inch. Any slight departure from a circumferentially balanced condition of flow and pressure of the entering injected material will exert lateral forces on the long, slim cantilevered core. These lateral forces cause the core to bend to an extent that makes the wall of the article unacceptably thin, or to an extent where the core contacts the side of the cavity and thus leaves a hole in the finished article.
Recognizing this problem in the molding of articles (even articles with thicker walls) the patent to Stillman 2,876,495 suggests a unique solution. For the molding of an article having a wall thickness as great as 0.060 inches, this patentee finds it necessary to employ a compex moving support to avoid the deflection due to lateral forces of fluid pressure exerted during injection. The patentee uses a sliding ring that supports the core from the mold cavity. The ring slides along the core and cavity under pressure of the entering injected fluid. The patent to Schultz 2,434,594 provides a similar sliding sleeve or ring to prevent deflection of the core and hold it concentric with the die cavity as the injected molding material fills the cavity. The ring is finally pushed off the core and into a recess surrounding the die member as the injection is completed. The arrangements of the patents to Stillman and Schultz are complex and bulky, and are subject to significant amounts of down time due to frequent mechanical breakdown of the slidable and moving parts. Further, the slidable member causes wear on the mold parts, thus changing part dimensions, venting may be uneven and not properly controlled and witness lines or undesirable flashing may occur.
In thermometer covers, where the article is to be inserted into a body cavity, it is essential that there be no roughness unevenness or other irregularity on the outer surface of the article and especially upon its tip. It is partly for this reason, that injection must occur at a point remote from the tip and that the core must be cantilevered, being entirely free at its tip to provide optimum smoothness at this portion of the finished article. Since many thermometer probes and accordingly, the probe covers or sheaths, are tapered or of varying cross section, the slidable ring or sleeve concept of the Shultz and Stillman patents cannot be employed, because these are useful only for articles of constant cross sectional configuration and dimension.
The significance of the problem of bending of a cantilevered core may be understood when it is noted that for an article having a wall thickness of 0.010 inches, deflection of the core tip of no more than 0.005 inches can be tolerated. This is so because the plastic cannot be forced to flow through a cavity space of less than 0.005 inches. Accordingly, if the core tip should deflect more than 0.005 inches in an arrangement for making articles having a nominal wall thickness of 0.010 inches, the article tip cannot be formed because no plastic can reach this area. Nevertheless, a typical steel core precisely positioned and aligned to be concentric with a cavity may deflect las much as 0.004 inches under forces exerted during conventional injection. This leaves the impossible tolerance of 0.001 inches for the total of all other factors that contribute to concentricity errors. Such other factors, in addition to the core deflection, include initial centering of the core mounting, actual relative dimensions of the core and cavity diameters, clearance of the shank of the core pin at the mounting and angle of the core pin axis (degree of perpendicularity of the core). All of these factors must total not more than 0.001 inches where a steel core pin is employed.
Another significant problem which contributes to the above-mentioned core deflection problem is adequate and symmetrically disposed venting. As the high pressure heated material is injected rapidly into the cavity space defined between the female cavity and the male mold core, air within the cavity space must be rapidly exhausted. Accordingly, vents are provided. Nevertheless, the vents must be of such size as to allow escape of air but to prevent or minimize flow of the plastic therethrough. Any flow of the plastic through the vent holes causes undesirable flash. Thus, venting passages sufficient to handle high rate of flow of the exhausting gases must be provided and yet the venting orifices communicating with the space within the cavity must be sufficiently small.
Balanced venting is an aspect that is of great importance in molding of thin wall articles. The venting must be precisely balanced about the cavity and about the core to ensure that venting of the gases does not take place more rapidly at one side of the core than the other. If the gas vents more rapidly at one side, that side will fill more rapidly. This may result in one side of the cavity, on one side of the core, containing high pressure liquid injected material before any balancing material has reached the other side of the core. Circumferentially unbalanced inflow of injected material will cause unacceptably large lateral and bending forces on the core and prevent the formation of an acceptable molded article.
Accordingly, it is an object of the present invention to enable molding of elongated thin wall articles.